Ignition damping box

C. Beaty;n1127462 said:
Since a flywheel magneto must supply enough energy at cranking speed to produce a spark, perhaps the energy at running speed is so much greater than battery ignition that the flyback pulse can produce arcing at the points.
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Chuck, Regardless of the energy of the magneto coil, the voltage not can exist on the primary side of one of the ignition coils before the corresponding breaker is open.
 
JC, when the points first open, the collapse of the magnetic field produces a spark at the spark plugs but the next ½ cycle, the backswing, can alxo be high voltage but of opposite polarity. The points will still be open.

Automobile ignition using coil and battery also produces a backswing but the higher the speed, the weaker the backswing since the current in the coil primary has less time to create a magnetic field.
 
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C. Beaty;n1127476 said:
JC, when the points first open, the collapse of the magnetic field produces a spark at the spark plugs but the next ½ cycle, the backswing, can alxo be high voltage but of opposite polarity. The points will still be open.

Chuck, I think that during the next ½ cycle, the points are already closed : Here, with a usual Dwell of 63%
Sans titre3.png
 
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JC, do you think the resonant frequency of the combination of condenser across the points and the inductance of ignition coil primary in parallel with the exciter coil is so low that a half cycle requires more time than the open time of the points?

If the condenser is 0.25 mfd and the inductance is 1 henry, the resonant frequency is 318 hz. The engine running at 5,000 rpm has 83 revolutions/second.

I don’t know the circuit inductance, I henry is only a wild guess.
 
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C. Beaty;n1127495 said:
I don’t know the circuit inductance, I henry is only a wild guess.

Very roughly 10 mH, Chuck. This gives a resonant frequency of about 3 kHz
 
Jean Claude;n1127497 said:
Very roughly 10 mH, Chuck. This gives a resonant frequency of about 3 kHz
In which case, the damper box totally suppresses oscillations of the ignition primary circuit; resonant oscillations can only be initiated by the opening of the breaker points. No other action has frequency components in the range of primary circuit resonance.
 
I have been trying to stay out of this but you have gotten the better of me...

The damper box circuit is related ONLY to the Magneto Coil.
A battery powered ignition does NOT have the Flyback issue that needs Dampening because there is no field collapsing in the non-existent magneto coil.

You do NOT want to Damp the flyback currents of the Spark Coils because the Collapsing Flyback currents are what produce the spark.
If you damped the Spark coils you would prevent the spark plugs from firing.

The Capacitor across the Points forms a resonant LC Circuit with the Spark Coil.
Using the 3khz and 5000 rpm example, A second or third spark might be produced every 10 degrees of rotation of the crank.
It is likely that the majority of the energy in the field was dissipated in the first spark and no actual additional sparks occur even though the oscillations exist in the circuit.
In either event, The fuel flame front from the first spark has moved outward and the additional sparks would add nothing to the combustion. Too Little and Too Late.
 
Bill,
3 khz is the example for this LC freely oscillating, ie when no spark has been initiate. So, the alternated voltage peak will produce each 1/2 cycle, ie each 4.5 degrees of rotation of crank if no spark.
But if the spark starts, then the energy disperses and the alternations disappears.
 
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This riddle could be solved instantly with an oscilloscope and a running 503/damper box.
 
It is unlikely that this will show the interest of install the damper box,
except by putting oneself in the unknown but destructive conditions that make it useful
 
I always wondered why rotax put that critter in there as cuyuna JLO, BSE and other engines with the Bosch energy transfer ignition system didn’t have it. I have run many hours without the box there without a problem.
norm
 
Here is what I believe now after much thought:
Capacitor and coil form an oscillating circuit, therefore several alternate sparks, every time. Only the first spark is useful for gas ignition while the nexts just does produce unnecessary heat on the electrode.
This possibly makes a soot point hot enough for self-ignition before the separation of the next round and severe loss of engine power (not to mention the severe mechanical overload of the elements)
The ignition damper box prevents this.
 
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I think there’s more to it than that, JC.

Battery ignition does not require a damper resistor because after points open, no more energy is being supplied to the ignition coil.

With a flywheel magneto, rotating magnets continue supplying energy to the ignition coil after opening of points.
 
Rotax 503 works as well without a spark damping box, like all other engines with flywheel magneto.
In my opinion, this box just is a cheap safety measure for use on aircraft.
 
t’s a mystery, JC, because most of the energy stored in the ignition coil is dissipated in its own internal resistance when the spark plug fires. I doubt if the back swing is significant.

It is different from turning off relays and solenoids.
 
Chuck,
In the case of the magnetic flywheel, the energy is stored in the internal coil. The external ignition coils are simply transformers. Their primary / secondary coupling is probably loose enough to allow a significant back swing, despite the first spark.
Sans titre.png
 
Of course the amplitude of back-swing voltage depends upon the amount of “leakage” inductance. I don’t have a Rotax engine in front of me but my recollection is that the ignition transformer has a complete magnetic circuit without an air gap.

Before “Kettering” single spark ignition was invented, early spark ignition engines used a “shower of sparks.” Model T Fords used ignition coils with a built in “buzzer” that made continuous sparks as long as power was supplied.
 
Well after seeing this topic discussed several times over the years in amazement !!
This damper is actually called a SNUBBER.
It's propose is to clamp FLYBACK caused from a collapsing inductor.
Since you have a stator and a transformer in series, the SNUBBER stops oscillations from occuring that may damage the points and keeps the coils from saturating after discharge.
The discharge and saturation rate will stay constant with the opening and closing of the points.
In other words ,it will only fire one time per saturation with less arc on the points.
All coils have a large flyback and that's what causes the high energy spark, but this also causes feedback in this ignition circuit.


rotax-bosch-ignition-wiring-diagram.jpg
 
The condenser prevents arcing at the points; slows down the rate of voltage increase when the points open, giving time for the points to separate.
 
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